1. Field of the Invention
The present invention relates to an apparatus using a laser beam, and more particularly to an apparatus, such as a laser beam printer, a facsimile, a copying machine, a display or the like, provided with a scanning device which scans a laser beam emitted from a laser source in a main scanning direction on a scanning surface in order to read, print or display an image. The present invention also relates to a scanning device which is provided in a laser beam printer, a facsimile, a copying machine, a display or the like to scan a laser beam emitted from a laser source on a scanning surface.
2. Description of Related Art
Recently, various kinds of laser beam scanning devices have been developed as an image reader, an image printer or an image projector of a laser printer, a facsimile, a copying machine, a display or the like. Such an image printer generally comprises a light source which is modulated in accordance with image data, a polygonal mirror which scans a laser beam emitted from the light source in a main scanning direction at a constant angular velocity, and an f.theta. system which adjusts the scanning speed in the main scanning direction. U.K. Patent No. 1,399,701 discloses that a cylindrical lens is provided between the light source and the polygonal mirror to converge a laser beam emitted from the light source on each reflective facet of the polygonal mirror in a sub scanning direction perpendicular to the main scanning direction. By converging the laser beam on each of the reflective facets of the polygonal mirror, a conjugate relationship is made between the reflective facets of the polygonal mirror and a scanning surface. Thereby, banding due to errors in the perpendicularity of the reflective facets can be prevented.
It is also well known to provide a plane mirror 30 between the light source and the cylindrical lens 31 as shown in FIG. 5. A laser beam Lb is deflected by the plane mirror 30, and the system from the light source to the polygonal mirror can be downsized. In FIG. 5, the line A indicates a converging line on which the laser beam is converged by the cylindrical lens 31.
The cylindrical lens 31 may be made of resin. Making the cylindrical lens 31 of resin has an advantage that mass production is possible only by preparing an accurate mold, thereby decreasing the cost. However, the refractive index of a resin lens changes with a change in temperature or in humidity, which causes a change in the focal length of the system. Consequently, defocus is caused, which brings about problems that the beam waist on the scanning surface becomes large and that the effect of rectifying errors in the perpendicularity of the reflective facets of the polygonal mirror is weakened.
In order to avoid these problems, it is possible to provide a mirror instead of the cylindrical lens as an element for converging the laser beam. However, in this case, the following problems occur. If a cylindrical mirror 32 is provided in the position of the plane mirror 30 as shown in FIG. 6, the converging line A will slant with respect to each reflective facet of the polygonal mirror, thereby causing defocus. This problem can be avoided by changing the relative positions of the cylindrical mirror 32 and the polygonal mirror, by providing a half mirror 33 after the cylindrical mirror 32 as shown in FIG. 7 or by constructing a three-dimensional optical path from the light source to the polygonal mirror as shown in FIG. 8 to make an angle in the sub scanning direction between an incident beam to the cylindrical mirror 32 and an emergent beam therefrom. However, either of the cases requires a large change to the arrangement of optical elements.